In industrial as well as domestic use, temperature control of domestic and/or heating water is very important. For example, a main boiler provided in many households for the central heating system is heated by a burner. A fuel/air mixture is fed to the boiler and the heat it generates is transferred to the main boiler via a heat exchanger. The supplied fuel can be gas. The firing-on and -off times for the heating boiler can be manually set with a timer so that heating water with a specified temperature can be made available for example in the morning and early evening. The boiler is well insulated, but as soon as the temperature of the burner boiler drops below a specified threshold value temperature, the burner is switched on via a simple on/off switching mechanism in order to increase the water temperature within the heating boiler. When the temperature of the boiler water has reached the predetermined and adjustable threshold temperature, an automatic switching off of the burner is effected.
In this heating system, temperature control takes place by means of an on/off control of the burner, which means that either the temperature of the water from the heating boiler is monitored and used for control of the on/off times of the burner, or the control of the on/off times is carried out via a detector mounted in a room to maintain the room temperature constant.
In such known heating systems, however, it is conceivable that the air/fuel mixture supply to the burner is controlled to such an extent that as few harmful substances as possible result from the combustion.
On the other hand, flow heaters are known for domestic water supply in which the application of a large quantity of energy to a small through-flow area in a domestic water supply line results in heating of domestic water when this goes through the supply line. These often include electrical flow heaters which use electrical heating coils for heating. In these, control does not normally ensue by means of the temperature of the domestic water, rather the predetermination of the temperature effects the control for the heating spools to feed a quantity of electrical energy corresponding to the predetermined target temperature.
For domestic water/central heating systems in the household, fuel/air mixture control systems are known for achieving an optimum boiler efficiency, as for example the "Gas-Air Ratio Control System for Optimum Boiler Efficency" described in the product information of Honeywell. Such a control system is shown in FIG. 4. This fuel/air mixture control system was especially developed to meet the requirements of clean and efficient use of heating boilers in the domestic area. Such a system makes control of the boiler efficiency possible over the entire operational range. In particular, it makes it possible to use energy always with the highest efficiency. It is also possible in such a system to provide a constant CO.sub.2 control or to control the CO.sub.2 values in the exhaust gas proportionally to the load. In FIG. 4, reference sign 16 denotes an air inlet to the burner, 17 a fuel inlet to the burner, 18 a differential pressure or Venturi valve, 2-2 a supplied air stream and 19 a consumer.
In this control system the direct gas flow to the burner is determined by the value of the differential pressure at the Venturi valve arrangement. The Venturi valve arrangement controls the outlet pressure proportionally to the differential pressure. Thus, the gas outlet pressure is controlled as a function of the differential pressure via a Venturi arrangement which is located in an air supply line. A special device transforms the detected air pressure difference into a gas outlet pressure. As FIG. 5 shows, this occurs at a pressure ratio of approximately 1 to 8. Additionally, this known system requires two pressure sensing lines 11-1, 11-2 and a transducer 11-3 for fuel/air control. The main function of the control system for a gas/air mixture shown in FIGS. 4 and 5 is to control the efficiency of the burner via the adjustable input load so that the harmful substances in the generated combustion gases do not exceed a preset value.
However, in a domestic water supply, the temperature of the water drawn from the boiler must be determined, i.e. a control of the gas/air mixture must be carried out in such a manner that the temperature of the water fed to a tap etc. is maintained constant. When little water is drawn off, only a small air/gas mixture must be supplied, whereas a large air/gas mixture must be supplied when a large quantity of water is used. This control must therefore operate in a wide modulation range for the air/gas mixture.
However, the air flow must be maintained constant for the gas modulation. A thermistor sensor can be arranged in the supply to the consumer and a potentiometer can be simplified in order to regulate the predetermined water temperature.
However, on account of the use of a Venturi valve arrangement, only modulation levels of the gas/air mixture in the range of typically 45% to 100% can be achieved. Thus, such a system is not suitable for temperature control for a domestic water supply which must cover a far greater temperature or modulation range. Additionally, such a system is of the on/off type so that an additional water mixing valve must be provided for the domestic water supply.
In addition to the disadvantage described above of not being able to control the domestic water supply and the fact that the shown arrangement is costly on account of the components used, strict safety requirements must obviously be met by such burner systems. This is especially important for the mass production of such control systems, as one can not expect that special safety precautions are always taken in mounting such control systems in many households. However, when a control system shown in FIGS. 4 and 5 is used, dangerous conditions can arise, as described in the following, i.e. the system does not have a fail-safe operation. This is so because the system uses two pressure sensing lines 11-1, 11-2 which monitor the differential, pressure of the air flow in the Venturi valve arrangement in the air supply line 16. If the pressure sensing line with low pressure, i.e. the downstream pressure sensing line has a leakage or is broken, the gas control valve is nevertheless opened on account of the incorrectly detected pressure difference and an increased gas supply to the burner is consequently effected. This excessive gas flow to the burner produces undesirable carbon monoxide on account of the insufficient air supply. This can cause a dangerous condition in the burner.
Additionally, the shown system is not cost effective. The system uses a transducer 11-3 for the control of the gas/air mixture which maintains the pressure ratio of 1 to 8 described above. The additional provision of a servo-regulator 11-4 thus increases the costs for the gas control.
Further, the influence of changes in ambient pressure can not be compensated for with the shown control system. For the servo-regulator 11-4 to be free of variations in ambient pressure, a combustion pressure compensation connection to the air-side (vent hole) of the gas control must be provided.
Summarizing, the above-described control systems for temperature control of burners have the following disadvantages:
a) The Venturi-valve arrangement controls at a ratio of differential pressure to burner pressure of 1:8; PA1 b) the air/fuel mixture can not be controlled in the range of 20% to 100% required for domestic water temperature control; PA1 c) a fail-safe operation can not be guaranteed; PA1 d) the number of required components is large and the control systems are therefore not cost-effective; and PA1 e) the control systems are dependent on ambient pressure variations. PA1 to provide a gas/air ratio control apparatus for a temperature control loop for gas appliances which enables control of the air/fuel mixture fed to a burner for a temperature range required for a domestic water supply, is cheap and allows fail-safe operation. PA1 a) a controllable fan for supplying a predetermined air stream to the burner in dependence on a detected actual temperature and a desired target temperature of the heating and/or domestic water; PA1 b) a pressure-controllable valve for controlling the supply of a specified fuel quantity to a burner exclusively in dependence on the absolute pressure of the air stream produced by the controllable fan; PA1 c) a pressure sensing line for transferring the absolute pressure of the air stream produced by the controllable fan to a control connection of the controllable valve; and PA1 d) two supply lines for the respective supply of the air stream and the fuel quantity to the burner with a nozzle arranged in the fuel supply line and a restriction arranged in the air supply line.
It is therefore the object of the invention